Cold cathode fluorescent lamps (CCFL) are widely used for backlights of large liquid crystal display (LCD) monitors and LCD TVs. FIG. 1 is a circuit diagram showing a conventional CCFL driving circuit as disclosed in Japanese Patent Application Laid-open Publication No. 1996-78180. As shown in FIG. 1, the CCFL driving circuit includes an inverter 100, a ballast capacitor 200, a sensing resistor 400, a voltage converting circuit 500, an error amplifier 600, a pulse width modulation (PWM) control circuit 700, and a discharge lamp 300. The inverter 100 converts a DC voltage of a DC power supply 110 to a high frequency voltage and supplies the high frequency voltage to the discharge lamp 300. The ballast capacitor 200 compensates for the negative impedance characteristic of the discharge lamp 300. The sensing resistor 400 senses a current flowing through the discharge lamp 300. The voltage converting circuit 500 performs a half-wave rectification on the voltage across the sensing resistor 400 to convert the voltage into a voltage of a pulse form. The error amplifier 600 generates a signal corresponding to the difference between an output signal of the voltage converting circuit 500 and a reference voltage. The PWM control circuit 700 compares an output signal of the error amplifier 600 with a reference signal of a triangle wave to output a pulse signal having a width varying with a lamp current.
In the LCD device, the periphery of a CCFL lamp is covered with a metal that is grounded, for protecting the CCFL lamp and decreasing electro-magnetic interference (EMI). However, a leakage current may flow through parasitic capacitors CPA existing between each terminal of the lamp and the metal cover 350. The amount of the leakage current may be equal to that of the lamp current. Because of the introduction of the grounded metal cover for decreasing the EMI, there may be a large difference between the current sensed by a sensing resistor 400 and the lamp current actually flowing through the discharge lamp 300.
Accordingly, there is a need for a discharge lamp driving circuit capable of detecting a lamp current accurately regardless of the metal cover introduced for decreasing the EMI. Further, there is a need for a discharge lamp driving circuit that does not operate when the lifetime of the discharge lamp is over, when there is no discharge lamp in the lamp driving system, or when the discharge lamp is not connected correctly. For designing such a discharge lamp driving circuit, there is a need to detect the voltage on a secondary side of a transformer.